How do I create low poly style terrain
Albeit the somewhat vague description of your problem, let's give it a try...

Creating low poly models as shown in the above image require a little more polygons to start with. The more data Blender has to work with, the better the result. 

Let's start with a nice terrain (in my case done with the ANT Landscape addon) that has a decent amount of geometry. My model has 16k faces.

 

Now go to Properties Window - Modifiers - Add Modifier - Decimate



In the Decimate modifier's settings chose "collapse", check "triangulate" and lower the ratio until you have the desired effect.



This is what your model should look like (reduced to 3k faces).